Noise cancellation circuit for a photomultiplier tube



United States Patent 01 fice Patented Mar. 11, 1969 3 Claims ABSTRACT OF THE DISCLOSURE A photomultiplier tube circuit which isolates the cathode current supply from the dynode power supply with a resistor substantially larger than component resistors in the dynode voltage divider and provides a filter capacitor only between the cathode and ground effectively eliminates the power supply ripple from the anode circuit of the photomultiplier tube. This occurs because the cathode to the first dynode noise voltage is approximately equal to N (where N is one less than the number of dynodes) times the magnitude of any other interdynode noise, and is of opposite polarity thereto, which polarity relatively provides substantial cancellation of any voltage supply ripple.

BACKGROUND OF THE INVENTION Field of invention This invention relates to an improved bias and filter circuit for a photomultiplier tube which substantially eliminates the power supply ripple from the output circuit of the tube.

Description of the prior art In some applications of a photomultiplier tube it is necessary to reduce power supply noise to a minimum, since such noise will appear in the output anode circuit. To reduce this noise it has been conventional practice to employ very large filter capacitors in the power supply. Because of the high voltages involved (in the region of 2000 volts) these capacitors, frequently as large as 3 mid. to achieve the necessary filtering, were extremely bulky and costly. In addition, when the photomultiplier tube is used in a servo system which adjusts the tube gain in accordance with the incident light to protect the tube from damage, the time constants of these very large filter capacitors were prohibitively long, resulting in destruction of the tube before the servo system could respond to reduce the supply voltage to a safe level.

Summary The instant invention obviates the use of very large filter capacitors in the power supply by isolating the photocathode from the dynode power supply path with a separate resistor of substantial resistance, and by providing a single filter capacitor between the cathode and ground. Because of the relatively small photocathode current, its filtering requirements can be achieved with a very small capacitor and a very large resistance, across which the voltage drop is negligibly small because of the small photocathode current.

It is, therefore, an object of this invention to provide an improved bias circuit for the elements of a photomultiplier tube to reduce the effect of power supply ripple upon the output current of the tube wherein the dynode elements of the tube are connected to respective taps on a voltage divider connected across the power supply, the photocathode is resistively coupled to the negative terminal of the power supply by a resistor substantially larger than the component resistors between taps on the voltage divider, and the photocathode is capacitively coupled to the positive terminal of the power supply.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.

In the drawing:

FIG. 1 is the sole drawing and shows the circuit diagram of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates as much of the circuit of a photomultiplier tube circuit as is necessary to an understanding of the principles of the invention. Here, the D.C. power supply 10 is voltage regulated by a feedback on line 11 from the output circuit of the photomultiplier tube 12. This power supply consists of :a variable amplitude square wave generator, a step-up transformer, and a full wave rectifier to provide a negative potential on line 13 of approximately 2000 volts with respect to ground. Because of the necessity of a fairly rapid response in the regulation of the voltage supply, it is not possible to employ the requisite filter capacitors in the filter network 14 to Smooth the ripples on the line 13, which ripples are aggravated by the fact that the input to the step-up transformer is a square waveform rather than a sine wave. Thus, the filter network consisting of the resistors R and R and capacitors C and C with the values marked on the drawing, are chosen so that their time constants are compatible with the required speed of the voltage regulation, while providing at least a basic filtering action. The D.C. supply on line 15, however, has an appreciable ripple. Independent of the design restrictions imposed by the requirement for voltage regulation, the savings in cost space of the filter elements is marked. It has been found that to achieve a filter action equivalent to that effected by the instant invention, filter capacitors of 8 mfd. capacity would necessarily be employed. With a 2000 volt working voltage, these are extremely bulky, as well as costly.

The negative D.C. supply on line 15 is coupled via resistor R to the photocathode 16 of the photomultiplier tube 12. Cathode 16 is capacitively coupled to ground via capacitor C It will be noted that R has a resistance of one megohm and C has a capacitance of .01 mfd. Because of the negligible current consumption (a few nanoamperes) of the cathode, the resistor R may be very large, while the filter capacitor C associated therewith may be very small, While still achieving the requisite filtering. Line 15 is further connected to the tapped voltage divider 18 which is returned to ground (positive with respect to line 15). The voltage divider 18 is tapped to provide the labelled bias voltages for each of the respective dynodes D through D with a power supply voltage of 2000 volts. It will be noted that except for dynodes D and D the potential difference between each successive pair of dynodes is 55 volts, which is also the potential difference between cathode 16 and dynode D The photomultiplier shield 19 is biased via R and R to a potential of -556 v., which resistors are serially connected in shunt with R in the dynode supply. The connection of anode 20 to ground via R provides the output response and the feedback on line 11 for voltage regulation; The foregoing voltage relationships obtain for a power supply potential of 2000 volts. Intense radiation of the tube will reduce this potential to as low as 50 volts together with a proportional reduction in the potentials throughout the circuit.

With the foregoing circuit connections and the labelled values of resistance, capacitance, and voltage levels, the apparatus operates with a noise level of 0.3 millivolt (peak to peak) on the cathode, 3.0 millivolts on dynode D but considerably less than 0.3 millivo-lt n the plate. In fact, the ripple noise on the plate was not measurable with ordinary instrumentation.

A qualitative explanation for the effective elimination of power supply noise in the output circuit is afforded by the fact that the noise voltage between the cathode and the first dynode is approximately N (where N equals one less than the number of dynodes in the tube) times the magnitude of the noise voltage between any adjacent pair of dynodes, and of opposite polarity with respect thereto. This phase opposition arises because the dynode current path is resistive while that for the cathode has the capacitive reactance present therein. This results in nearly perfect cancellation of the noise inherent in an imperfectly filtered power supply. In fact it provides, as above described, a very compact and inexpensive substitute for the filter network that would otherwise be required in the basic power supply circuit.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An improved photomultiplier tube circuit for providing substantial cancellation of the alternating current ripple in the direct current power supply from the output circuit of the tube, which tube is disposed in a circuit with its dynodes connected to seriately arranged taps on a voltage divider connected across the output terminals of the power supply to provide the requisite bias potentials on the respective dynodes, wherein the improvement comprises:

(a) a separate resistor connecting the cathode of the photomultiplier tube to the negative terminal of the power supply to thus isolate the cathode from the dynode voltage divider path;

(b) and a capacitor connected between the cathode and the positive terminal of the power supply, the

said capacitor being the sole passive reactance element in the circuit of the photomultiplier tube between the output terminals of the power supply, whereby the phase of the alternating ripple voltage of the first dynode relative to the cathode is reversed with respect to the ripple voltage between any two adjacent dynodes, and substantially equal to the interdynode voltage multiplied by one less than the number of dynodes, to thus provide substantial cancellation of the ripple in the anode circuit of the tube.

2. The improvement of the circuit of claim 1 wherein the ratio of the resistance of the said resistor to the capacitive reactance of the said capacitor at the basic frequency of the alternating current ripple in the direct current power supply is greater than fifty.

3. A circuit for substantially cancelling from the output circuit of a photomultiplier tube the alternating current ripple component of an imperfectly filtered direct current supply, comprising:

(a) a tapped resistor connected across the output terminals of the power supply with the dynodes of the photomultiplier tube connected seriately to the top thereof to provide the requisite bias potentials therefor;

(b) a resistor connecting the anode of the tube to the positive terminal of the power supply;

(c) a separate resistor having a substantially higher resistance than the resistance of said tapped resistor connecting the cathode of said tube with the negative terminal of the power supply; and

(d) a capacitor connecting the cathode of said tube to the positive terminal of the power supply, the said capacitor being the sole passive reactance element in circuit between the positive and negative terminals of the power supply, whereby the reactance in the cathode circuit provides a phase reversal in the cathode circuit with respect to the dynode circuits to provide an effective cancellation of the ripple in the anode circuit.

References Cited UNITED STATES PATENTS 2,141,322 12/1938 Thompson 250207 X 2,798,165 7/1957 Neher 250-207 2,958,785 11/1960 Camp 250-207 X 3,089,959 5/1963 Chatten 250207 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R. 

